In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and lithographic printing ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is eventually transferred to the surface of a material upon which the image is to be reproduced.
Lithographic printing precursors useful for preparing lithographic printing plates or sleeves typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is considered as positive-working. Conversely, if the non-imaged regions are removed, the precursor is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital imaging has become increasingly important in the printing industry. Lithographic printing precursors for the preparation of lithographic printing plates have been developed for use with infrared lasers that image in a platesetter in response to signals from a digital copy of the image in a computer. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the precursors.
Positive-working imageable compositions containing one or more phenolic or vinyl acetal polymeric binders have been used in positive-working lithographic printing plate precursors for many years. For example, such imageable compositions are described in U.S. Pat. Nos. 7,399,576 (Levanon et al.) and 7,544,462 (Levanon et al.) and U.S. Patent Application Publications 2006/0154187 (Wilson et al.) and 2009/0162783 (Levanon et al.). However, there is a continuing need to improve their resistance to certain press chemicals and solvents.
It was found that solvent resistance could be improved using a poly(vinyl acetal) that also includes recurring units having hydroxyaryl ester groups, as described for example, in copending and commonly assigned U.S. Ser. No. 12/555,040 (filed Sep. 9, 2009 by Levanon, Bylina, Kampel, Rubin, Postel, Kurtser, and Nakash). While good run length and solvent resistance were obtained with these plates, there was a continued need to improve the development latitude in commonly-used developers.
It is quite common to design a specific developer composition that is optimized for developing a particular positive-working lithographic printing plate precursor. There have been attempts to do this by including coating protecting agents in the developer to reduce the solubility of the imageable coating in the non-exposed areas more effectively than the imageable coating in the exposed areas.
One of the common causes of short development cycle and excessive difficulty in cleaning the automatic processor relates to partial dissolution of aluminum oxide film on the substrate of typical lithographic printing plate precursors in the developer solution. Known techniques for reducing or eliminating such aluminum oxide attacks include the use of alkali silicates, non-reducing sugars, or lithium salts such as lithium chloride. However, the use of silicate salts itself adds to the dirtiness of the processor bath. It was found that developers containing lithium chloride are very slow in dissolving the infrared laser exposed coating containing polyvinyl acetal that also has hydroxyaryl ester groups and therefore are considered unsuitable for processing such precursors.
These problems are addressed using the method described and claimed in copending and commonly assigned U.S. application Ser. No. 12/948,808 filed on even date herewith by Levanon, Huang, and Askadsky and entitled METHODS OF PROCESSING USING SILICATE-FREE DEVELOPER COMPOSITIONS). Improved image discrimination was achieved with the described lithographic printing plate precursors by processing them using a developer composition having a pH of at least 12 and comprising at least 0.001 gram-atom/kg of a metal cation M2+ such as barium, calcium, strontium, and zinc cation.
The presence of M2+ cations such as calcium ions in the developer composition can also act to protect the aluminum substrate from attack by the alkaline developer solution.
During a processing cycle, when the developer is “loaded” with dissolved coating materials, a problem known as “sharpening” can become evident. “Sharpening” results from a change in the developer composition to be more aggressive in its developing activity so that non-exposed regions in the imageable layer are attacked by the developer, resulting in increased printing plate weight loss and decreased dot size (dot sharpening) in the resulting printed images.
These problems were addressed in our copending and commonly assigned U.S. application Ser. No. 12/948,812 (filed on even date herewith by us and entitled SILICATE-FREE DEVELOPER COMPOSITIONS.
However, there has been a desire in the industry to reduce or eliminate the presence of silicates and metasilicates in the developer compositions to increase cycle length and to improve processor cleanliness. While silicate-free developer compositions are known in the art, they are prone to increased attack of aluminum oxide coatings on the substrates. Calcium ions have then been used to reduce this problem but calcium salts and calcium hydroxide precipitation often occurs in the developer composition. Therefore, common chelating agents such as ethylenediaminetetraacetic acid (EDTA) and its salts have been added to the developer compositions. However, we have found that while many of these common chelating agents are effective to prevent calcium salt and hydroxide precipitation, but the same chelating agents also may reduce the effectiveness of calcium cations to protect the aluminum substrates and even increase the attack of the aluminum oxide by hydroxide-based developer composition containing non-optimized amounts of the chelating agents.
There is a desire, then, to provide silicate-free developer compositions containing chelating agents that do not promote the attack of the substrate over a very broad concentration window, and in which no precipitation of calcium or other cations used to protect the substrate occurs.